Exhaust gas purifying method and exhaust gas purifying system

ABSTRACT

In an exhaust gas purifying system ( 1 ) for applying NOx purification by a NOx occlusion reduction type catalyst ( 42 ) and PM purification by a DPF ( 41 ) to the exhaust gas of an internal combustion engine, when it is judged that both regeneration of the DPF ( 41 ) and sulfur purge of the NOx occlusion reduction type catalyst ( 42 ) are necessary, the DPF regeneration control for raising the temperature of the DPF ( 41 ) is performed and the sulfur purge control for decreasing the oxygen concentration of the exhaust gas flowing into the NOx occlusion reduction type catalyst ( 42 ) is intermittently repeated. Thereby, it is possible to efficiently purge the sulfur accumulated in the NOx occlusion reduction type catalyst while preventing deterioration of fuel efficiency and discharge of NOx, HC, and CO into atmospheric air.

BACKGROUND OF THE INVENTION

The present invention relates to an exhaust gas purifying method and anexhaust gas purifying system for purifying NOx by a NOx occlusionreduction type catalyst and purifying PM by a DPF.

Legal restriction on discharge quantities of particulate matter(hereafter referred to as PM) and NOx (nitrogen oxide) is enforced yearby year together with legal restriction on discharge quantities of CO(carbon monoxide) and HC (carbon hydride). Thus, only improvement of anengine cannot manage a restriction value for the enforcement of therestriction. Therefore, techniques are adopted which reduces thesematters discharged from an engine by mounting an exhaust gas controlsystem.

A filter referred to as a diesel particulate filter (hereafter referredto as DPF) is developed for the PM and many NOx purifying catalysts aredeveloped for NOx.

When purifying PM and NOx simultaneously, it is impossible to avoid theNOx flowing out by the DPF alone and to burn a soot component in PM bythe NOx occlusion reduction type catalyst alone. Therefore, as disclosedin Japanese Patent Laid-Open No. 1997-53442, it is required to combinethe catalyst with a DPF or integrate the NOx purifying ability of theNOx occlusion reduction type catalyst with the PM purifying ability ofthe DPF. Moreover, it is required to combine both in order to purify theNOx generated in the time of regeneration of the DPF.

This DPF frequently uses a wall-flow-type ceramic honeycomb structuremainly containing cordierite and silicon carbide. The wall-flow-type DPFis formed by having a plurality of cells (through-holes) divided byporous partitions. Moreover, the DPF is alternately closed like acheckered pattern at the exhaust gas inlet-side end and exhaust gasexit-side end. Furthermore, the exhaust gas passes through the porouspartitions of the cells when it moves from a cell whose upstream side isopened and whose downstream side is closed to the next cell whoseupstream side is closed and whose downstream side is opened. When theexhaust gas passes through the porous partitions, PM in the exhaust gasis caught in the partition portion and the exhaust gas is purified.

However, when the PM is accumulated in the porous partitions of thecells, clogging occurs. Therefore, the ventilation resistance increases,and the collection efficiency of the PM is deteriorated. Therefore, toremove the PM accumulated in the porous partitions of the DPF byburning, when it is judged that the quantity of the collected PM exceedsa predetermined accumulated quantity, the temperature of the DPF israised to the PM combustion start temperature or higher to remove the PMby burning.

The method for raising the temperature of a DPF to remove PM by burningincludes an exhaust gas temperature raising method by performing postinjection in the control of injecting fuel into a cylinder, an exhaustgas temperature raising method by directly injecting hydrocarbon (HC)into an exhaust passage, and a current-carrying-heating method using anelectric heater set in a DPF.

Furthermore, in order to remove PM by burning even in a state where theexhaust gas temperature is comparatively low, a continuouslyregenerating type DPF is developed and proposed which is constituted bycombining an oxidation catalyst or the like with the DPF. The DPF canremove the PM by burning at a comparatively low temperature. However, ina state where an exhaust gas temperature is low and clogging of the DPFprogresses, exhaust gas temperature raising control such as anintake-air throttling is performed to temporarily raise the exhaust gastemperature in order to remove the collected PM by burning.

A NOx occlusion reduction type catalyst is one of the NOx purifyingcatalysts. This catalyst shows a NOx occlusion ability, and a NOxrelease and purification ability of depending on the 02 (oxygen)concentration in the exhaust gas. In the NOx occlusion reduction typecatalyst, a catalyst metal having an ability to oxidize NOx and a NOxocclusion material having an ability to occlude NOx are supported on aporous catalyst coat layer such as alumina (Al₂O₃). The catalyst metalis formed by platinum (Pt), palladium (Pd) and so on. The NOx occlusionmaterial is formed by any one or several of alkaline metals, alkalineearth metals, rare earths and the like. As alkaline metals includessodium (Na), potassium (K), cesium (Cs), and so on. The alkaline-earthmetals include calcium (Ca), barium (Ba) and so on. The rare-earthsinclude yttrium (Y), lanthanum (La) and so on.

First, in the case of an exhaust gas condition in which O₂ (oxygen)concentration in the exhaust gas is high (lean air-fuel ratio (air-fuelratio) state) as in the normal operational state of a diesel engine,lean-burn gasoline engine, NO (nitrogen monoxide) is oxidized by O₂contained in the exhaust gas as a result of the oxidizing ability of thecatalyst metal to become NO₂ (nitrogen dioxide). Since the NO₂ isoccluded by the NOx occlusion material in the form of nitrate, theexhaust gas is thus purified.

However, when this occlusion of the NOx continues, the NOx occlusionmaterial such as barium is changed to nitrate. Accordingly, the NOxocclusion material is gradually saturated to lose the ability foroccluding NOx. To avoid such situation, over-rich combustion isperformed by changing operation conditions of the engine to generate theexhaust gas (rich spike gas) having a low O₂ concentration, high COconcentration, and high exhaust gas temperature and supply the exhaustgas to the catalyst.

In the rich/air ratio state of the exhaust gas, the NOx occlusionmaterial changed to nitrate by occluding NO₂ releases the occluded NO₂and returns to the original substance such as barium. Because O₂ is notpresent in this exhaust gas, the released NO₂ is reduced on the catalystmetal by using CO, HC, and H₂ in the exhaust gas as reducers. Thus, theNOx is converted into N₂, H₂O, and CO₂, and purified.

The NOx occlusion reduction type catalyst has a problem in that sulfur(sulfur component) in fuel is accumulated in the NOx occlusion material,and the NOx purifying efficiency is deteriorated as the operation of theengine continues. Therefore, as disclosed in Japanese Patent Laid-OpenNo. 2000-192811, it is required to perform sulfur purge control(desulfurization control) by keeping the exhaust gas flowing into thecatalyst in the condition of a high temperature and a rich air-fuelratio atmosphere. Though different depending on the types of thecatalyst to be used, the temperature of the exhaust gas is set higherthan approximately 600° C. to 650° C.

In the case of the sulfur purge control of a diesel engine, the richair-fuel ratio state is realized by reducing the exhaust gas volumethrough intake-air throttling or through a large quantity of EGR as wellas by a post injection or by directly adding light oil to an exhaustpipe. Sulfur purge is accelerated by bringing the exhaust gas into therich air-fuel ratio state and raising the temperature of the catalyst bythe oxidation activation reaction heat of the catalyst.

However, the sulfur purge for recovering the NOx occluding ability ofthe catalyst by increasing the sulfur purge quantity has the followingproblems.

Because the oxygen concentration in the exhaust gas is very low under arich air-fuel ratio state, the time required to raise the temperature ofthe catalyst up to the temperature at which the sulfur purge can berealized becomes very long. Therefore, the fuel consumption amountduring that time is increased, and the fuel efficiency is deteriorated.Moreover, the denser is an air-fuel ratio sate of the exhaust gas, themore a sulfur purge quantity increases. However, when performing such adense rich air-fuel ratio state operation, there are problems that thefuel efficiency is extremely deteriorated, and that exhaust gascomponents such as HC, CO are generated in a large quantity. Moreover,when the oxygen concentration in exhaust gas becomes 0% at a hightemperature, a problem occurs that hydrogen generated by a hydrogengeneration reaction is combined with sulfur to change harmful hydrogensulfide (H₂S).

The sulfur purge has the two problems; the first problem is to easilydischarge purged sulfur in large quantity in an initial period of thesulfur purge control, and the second problem is one that a long timeholding of the air-fuel ratio state of the exhaust gas in astoichiometric air-fuel ratio state is difficult in the diesel engine.

Therefore, sulfur purge control sets a rich air-fuel ratio state when acatalyst temperature rises to a temperature at which sulfur separationcan be made or higher. However, it is preferable to set the richair-fuel ratio state in a minimum time.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide an exhaust gaspurifying method and an exhaust gas purifying system capable ofefficiently purging the sulfur accumulated in a NOx occlusion reductiontype catalyst while preventing the fuel efficiency from deterioratingand NOx, HC, and CO from being discharged into atmosphere, in an exhaustgas purifying system constituted by combining the NOx purifying abilityof a NOx occlusion reduction type catalyst with the PM purifying abilityof a DPF.

In order to achieve the above described object, the exhaust gaspurifying method of the invention using an exhaust gas purifying systemhaving a DPF and a NOx occlusion reduction type catalyst arranged inorder from the upstream side in an exhaust passage of an engine, and thecontrol means which is comprising an exhaust gas component detectionmeans to detect the oxygen concentration and NOx concentration of theexhaust gas passing through the NOx occlusion reduction type catalyst, aDPF regeneration control means for controlling regeneration of the DPF,a NOx catalyst regeneration control means for recovering the NOxocclusion ability of the NOx occlusion reduction type catalyst, and asulfur purge control means for controlling the sulfur purge of the NOxocclusion reduction type catalyst, wherein, when it is judged that bothregeneration of the DPF and sulfur purge of the NOx occlusion reductiontype catalyst are necessary, the DPF regeneration control for raisingthe temperature of the DPF is performed and the sulfur purge control todecrease the oxygen concentration in the exhaust gas flowing into theNOx occlusion reduction type catalyst is intermittently repeated.

This sulfur purge control is not set to the one-time period of sulfurpurge control to be continued until the sulfur purge is completed. Butthe one-time period of the sulfur purge control is set to one several-thof the time required to complete the sulfur purge (e.g. 180 s to 300 s)or 2 s to 60 s and the short period sulfur purge is repeatedly performeduntil the sulfur purge is completed at predetermined time intervals(equal intervals or unequal intervals). It is allowed to set thepredetermined time interval to a constant time. For example, it isallowed not to perform the sulfur purge control when the temperature ofa NOx occlusion reduction type catalyst is equal to or lower than apredetermined temperature but to change the predetermined time intervalin accordance with the temperature of the NOx occlusion reduction typecatalyst when the temperature of it is higher than the predeterminedtemperature. It is possible to predetermine the one-time period of thesulfur purge by considering the conditions in which sulfur purge can beefficiently performed. These conditions are previously obtained throughexperiments and the like.

Thereby, it is avoided that a lot of sulfur is released and dischargedat the beginning of sulfur purge control. Moreover, the time for astoichiometric air-fuel ratio state which is difficult to keep for along time in the case of a diesel engine, is decreased. Furthermore, itis possible to avoid a lot of sulfur from being temporarily produced inaccordance with the intermittent sulfur purge control. Furthermore, theslip (discharge to atmospheric air) of HC, CO, H₂S and the deteriorationof drivability are constrained.

Furthermore, it is preferable to determine the air-fuel ratio state ofexhaust gas in the sulfur purge control in accordance with the air-fuelratio state of the exhaust gas flowing into the NOx occlusion reductiontype catalyst. It is assumed that the air-fuel ratio state of theexhaust gas is a rich air-fuel ratio or preferably, a stoichiometricair-fuel ratio (theoretical air-fuel ratio).

That is, in the sulfur purge control, the air-fuel ratio state of theexhaust gas flowing into the NOx occlusion reduction type catalyst isbrought into a stoichiometric air-fuel ratio state.

Moreover, an exhaust gas purifying system for achieving the abovedescribed object having a DPF and a NOx occlusion reduction typecatalyst arranged in order from the upstream side in an exhaust passageof an engine, and the control means which is comprised of an exhaust gascomponent detection means to detect the oxygen concentration and NOxconcentration of the exhaust gas passing through the NOx occlusionreduction type catalyst, a DPF regeneration control means forcontrolling regeneration of the DPF, a NOx catalyst regeneration controlmeans for recovering the NOx occlusion ability of the NOx occlusionreduction type catalyst, and a sulfur purge control means forcontrolling the sulfur purge of the NOx occlusion reduction typecatalyst, wherein when it is judged that both regeneration of the DPFand sulfur purge of the NOx occlusion reduction type catalyst arenecessary, the control means executes intermittently the sulfur purgecontrol to decrease the oxygen concentration in the exhaust gas flowinginto the NOx occlusion reduction type catalyst, performing DPFregeneration control for raising the temperature of the DPF.

The air-fuel ratio state of the exhaust gas flowing into the NOxocclusion reduction type catalyst is preferably brought into astoichiometric air-fuel ratio state in the sulfur purge control.

According to an exhaust gas purifying method and an exhaust gaspurifying system of the present invention, DPF regeneration according toforcible combustion of PM and sulfur purge of a NOx occlusion reductiontype catalyst are simultaneously performed in an exhaust gas purifyingsystem used by combining a DPF with a NOx occlusion reduction typecatalyst. Therefore, it is possible to raise the temperature of the NOxocclusion reduction type catalyst by using the heat generated due toforcible combustion of the PM. Therefore, it is possible to minimize thedeterioration of fuel efficiency.

Moreover, by intermittently repeating the sulfur purge control, it ispossible to prevent a lot of sulfur from being released and dischargedat the time of the sulfur purge control and eliminate the necessity forkeeping the air-fuel ratio state of exhaust gas in a stoichiometricair-fuel ratio state for a long time.

Furthermore, because the sulfur purge control for bringing the air-fuelratio state of exhaust gas into a stoichiometric air-fuel ratio state isintermittently performed, it is possible to efficiently perform sulfurpurge while restraining slip (discharge to atmospheric air) of HC, CO,H₂S, and the like while preventing the drivability form deterioration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a constitution of an exhaust gas purifying system of anembodiment of the present invention;

FIG. 2 shows a constitution of an exhaust gas purifying apparatus of thefirst embodiment of the present invention;

FIG. 3 shows a constitution of an exhaust gas purifying apparatus of thesecond embodiment of the present invention;

FIG. 4 shows a constitution of an exhaust gas purifying apparatus of thethird embodiment of the present invention;

FIG. 5 shows a constitution of an exhaust gas purifying apparatus of thefourth embodiment of the present invention;

FIG. 6 shows a control flow for a sulfur purge of an exhaust gaspurifying method of an embodiment of the present invention; and

FIG. 7 shows a time series of the excess air factor, differentialpressure between the front and the rear of a DPF, the temperature of aNOx occlusion reduction type catalyst converter, a NOx concentration,and a SO₂ concentration of an embodiment using a control flow for asulfur purge of an exhaust gas purifying method of an embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An exhaust gas purifying method and an exhaust gas purifying systems ofembodiments of the present invention are described below by referring tothe accompanying drawings.

FIG. 1 shows a constitution of the exhaust gas purifying system 1 of theembodiment. The exhaust gas purifying system 1 is constituted an exhaustgas purifying apparatus 40A constituted by arranging an oxidationcatalyst (DOC) 41 a, a DPF 41 b, and a NOx occlusion reduction typecatalyst converter 42 in order from the upstream side of an exhaustpassage 20 in an engine (internal combustion engine) E. Moreover, acontinuously regenerating type DPF 41 is constituted of theupstream-side oxidation catalyst 41 a and the downstream-side DPF 41 b.

The oxidation catalyst 41 a is formed by a monolithic catalyst having alot of polygonal cells made of structural material of cordierite, SiC(silicon carbide), or stainless steel. A catalyst coat layer occupyingthe surface area is present in inner walls of the cells to make thelarge surface support a catalyst metal such as platinum or vanadium andthe ability of catalyst is generated. Thereby, it is possible to changeNO in the exhaust gas to NO₂ by an oxidation reaction (NO+O→NO₂).

Moreover, the DPF 41 b can be formed by a monolith-honeycomb wall-flowfilter obtained by alternately sealing inlets and exits ofporous-ceramic honeycomb channels or a felt-like filter obtained bylaminating inorganic fibers of alumina or the like at random. Suchfilter collects the PM in the exhaust gas. By combining the PM with theupstream front-stage oxidation catalyst 41 a, the collected PM is burnedby NO₂ having a high oxidative power and removed.

The NOx occlusion reduction type catalyst converter 42 is formed by amonolithic catalyst similarly to the oxidation catalyst 41 a. A catalystcoat layer is formed on the support body such as aluminum oxide ortitanium oxide to make the catalyst coat layer support a noble metalsuch as platinum and a NOx occlusion material (NOx occlusion substance)such as barium.

The NOx occlusion reduction type catalyst converter 42 purifies the NOxin the exhaust gas by occluding the NOx in the exhaust gas when oxygenconcentration in the exhaust gas state is high (lean air-fuel ratiostate). The NOx occlusion reduction type catalyst converter 42 releasesthe occluded NOx and reduces the released NOx when oxygen concentrationin the exhaust gas is low or zero (rich air-fuel ratio state). Thereby,it is prevented that NOx is discharged into the atmosphere.

The first temperature sensor 51 and the second temperature sensor 52 areprovided on the upstream side and the downstream side of the DPF 41 brespectively. Furthermore, the fist exhaust gas concentration sensor 53and the second exhaust gas concentration sensor 54 are provided on thefront and the rear of the NOx occlusion reduction type catalystconverter 42, that is, nearby the inlet and the exit of the exhaust gaspurifying apparatus 40A in FIG. 1. The exhaust gas concentration sensors53 and 54 are the integrator sensors in which a λ (excess air factor)sensor, a NOx concentration sensor, and O₂ concentration sensor areintegrated. Moreover, to estimate a deposition quantity of PM, adifferential pressure sensor 55 for detecting an exhaust differentialpressure ΔP between the front and the rear of the DPF is provided with aconduction pipe connected to the front and the rear of the DPF 41 b(FIG. 1) or the front and the rear of the exhaust gas purifyingapparatus 40A (FIG. 2).

Output values of these sensors are input to a control unit (ECU: enginecontrol unit) 50 which performs the overall control of operations of theengine E and performs the regeneration control of the continuouslyregenerating type DPF 41 and the recovery control of the NOxpurification capacity of the NOx occlusion reduction type catalystconverter 42. Moreover, a common-rail electronic-control fuel-injectionsystem for fuel injection of the engine E, a throttle valve 15, an EGRvalve 32, and the like are controlled depending on control signalsoutput from the control unit 50.

The control unit 50 calculates a NOx purifying rate RNOx(=1.0-CNOx2/CNOx1) based on the values CNOx1 and CNOx2 detected by thefirst and second exhaust gas concentration sensors 53 and 54.Furthermore, the PM accumulation quantity of the DPF 41 b is estimatedbased on the differential pressure ΔP detected by the differentialpressure sensor 55 or the like.

In the exhaust gas purifying system 1, air A passes through an aircleaner 11 and a mass air flow (MAF) sensor 12, a compressor 13 a of aturbocharger 13, an intercooler 14, and a throttle valve 15 in an intakepassage 10 and enters into the cylinder of the engine through an intakemanifold 16. The quantity of the air A is adjusted by a throttle valve15.

Moreover, the exhaust gas G generated in the cylinder flows out from anexhaust manifold 21 and drives a turbine 13 b of the turbocharger 13 inan exhaust passage 20. And the exhaust gas G passes through the exhaustgas purifying apparatus 40A and a not-illustrated silencer to dischargeinto the atmosphere. Then, the exhaust gas G becomes purified exhaustgas Gc in the exhaust gas purifying apparatus 40A.

Furthermore, some of the exhaust gas G passes through an EGR cooler 31of an EGR passage 30 and an EGR valve 32 as ERG gas. This gas isre-circulated into the intake manifold 16. The quantity of EGR gas Ge iscontrolled by an EGR valve 32.

FIG. 2 shows the exhaust gas purifying apparatus 40A. FIGS. 3 and 4 showconstitutions of the exhaust gas purifying apparatuses 40B and 40C ofother embodiments. The exhaust gas purifying apparatus 40B in FIG. 3 isconstituted of the oxidation catalyst 41 a and the DPF 43 supporting aNOx occlusion reduction type catalyst. The exhaust gas purifyingapparatus 40C in FIG. 4 is constituted of the oxidation catalyst 41 aand the DPF with the catalyst 44 supporting the NOx occlusion reductiontype catalyst. The DPF with the catalyst includes the DPF supporting anoxidation catalyst and the DPF supporting an oxidation catalyst and thePM oxidation catalyst.

The PM oxidation catalyst is made of the oxide of cerium (Ce) or thelike. In the case of the catalyst-carrying filter carrying both of thePM oxidation catalyst and the oxidation catalyst, PM is oxidizeddepending on a reaction (4CeO₂+C→2Ce₂O₃+CO₂, 2Ce₂O₃+O₂→4CeO₂, or thelike) using O₂ in the exhaust gas in the catalyst-carrying filter at alow temperature (between 300° C. and 600° C.). The PM is oxidized by O₂in the exhaust gas at a temperature higher than the temperature (600° C.or higher) at which the PM is burned by O₂ in the exhaust gas.

Moreover, there are apparatuses as an exhaust gas purifying apparatushaving no oxidation catalyst at the most upstream side, such as theexhaust gas purifying apparatus constituted of a DPF not having acatalyst but having only a filter and a NOx occlusion reduction typecatalyst converter; the exhaust gas purifying apparatus constituted of aDPF with a catalyst carrying an oxidation catalyst and a NOx occlusionreduction type catalyst converter; and the exhaust gas purifyingapparatus DPF with a catalyst supporting both an oxidation catalyst anda PM oxidation catalyst and a NOx occlusion reduction type catalystconverter.

In short, any exhaust gas purifying apparatus may be used as the exhaustgas purifying apparatus of the present invention as long as theapparatus performs NOx purification by the NOx occlusion reduction typecatalyst and PM purification by the DPF to the exhaust gas of theengine.

Moreover, the control unit of the exhaust gas purifying system 1 isbuilt in the control unit 50 of the engine E. The control unit 50controls operations of the engine E and the exhaust gas purifying system1. As shown in FIG. 5, the control unit of the exhaust gas purifyingsystem 1 is constituted of a control means Cl of the exhaust gaspurifying system having an exhaust gas component detecting means C10, aDPF control means C20, and a NOx occlusion reduction type catalystcontrol means C30.

The exhaust gas component detecting means C10 is a means for detectingthe oxygen concentration (or excess air factor λ) and the NOxconcentration in the exhaust gas and is constituted of the first andsecond exhaust gas concentration sensors 53 and 54.

The DPF control means C20 is constituted of a PM accumulation quantitycalculating means C21, a DPF regeneration start judgment means C22, anda DPF regeneration control means C23.

In the DPF control means C20, is performed the following. The PMaccumulation quantity calculating means C21 calculates the PMaccumulation quantity of the DPF 41 b based on the differential pressureΔP detected by the differential pressure sensor 55, or the like. The DPFregeneration start judgment means C22 judges whether the clogging stateof the DPF 41 b exceeds a predetermined clogging state depending onwhether the PM accumulation quantity exceeds a predetermineddetermination value. When DPF regeneration start is judged, the DPFregeneration control means C23 raises an exhaust gas temperature throughpost injection, EGR control, and the like, and the DPF 41 isregenerated.

The NOx occlusion reduction type catalyst control means C30 is a meansfor regenerating the NOx occlusion reduction type catalyst converter 42and controlling a sulfur purge and is constituted of a regenerationstart judgment means of NOx catalyst C31, a NOx catalyst regenerationcontrol means C32, a sulfur purge start judgment means C33, and a sulfurpurge control means C34.

The NOx occlusion reduction type catalyst control means C30 calculates aNOx purification rate RNOx based on the NOx concentration detected bythe exhaust gas component detecting means C10. Moreover, when the NOxpurification rate RNOx becomes lower than a predetermined determinationvalue, the means C30 judges that regeneration of the NOx catalyst isstarted. An exhaust gas state is brought into a predetermined richair-fuel ratio state and a predetermined temperature range (betweenapproximately 200° C. and 600° C. though depending on a catalyst) byperforming post injection in the fuel injection control of the engine E,EGR control, and intake-air throttling control by the NOx catalystregeneration control means C32. Thereby, the NOx purification capacity,that is, the NOx occlusion capacity is recovered and the NOx catalyst isregenerated. Moreover, the sulfur purge is performed by the sulfur purgestart judgment means C33 and the sulfur purge control means C34.

In such exhaust gas purifying system 1, the exhaust gas purifying methodof NOx occlusion reduction type catalyst of the present invention isperformed in accordance with the sulfur purge control flow shown in FIG.6.

The control flow shown in FIG. 6 is a control flow relating to thesulfur purge of the NOx occlusion reduction type catalyst. The controlflow is shown as a flow for being repeatedly called from the controlflow of the whole exhaust gas purifying system together with the controlflow relating to the regeneration of the NOx occlusion capacity of theNOx occlusion reduction type catalyst converter 42 or the regenerationcontrol flow of the DPF 41 b, or the like. The above control flow isperformed for judging the necessity of the sulfur purge and the DPFregeneration, and if required, the sulfur purge control isintermittently performed at the same time as the regeneration control ofthe DPF.

When this control flow starts, the sulfur quantity occluded in thecatalyst 42 is calculated in accordance with the fuel consumption andthe sulfur quantity contained in fuel by the sulfur purge start judgmentmeans C33 and integrated to calculate an accumulated sulfur quantitySsp. Moreover, the accumulated PM quantity PMst of the DPF 41 b iscalculated in accordance with the pressure difference ΔP detected by thePM accumulation quantity calculating means C21.

Then, in the next step S12, it is judged by the sulfur purge startjudgment means C33 whether sulfur purge is necessary. In the case ofthis judgment, it is judged that sulfur purge is necessary when theaccumulated sulfur quantity Ssp becomes larger than a predeterminedlimit value Sso0.

Moreover, in step S12, it is judged by the DPF regeneration startjudgment means C22 whether DPF regeneration is necessary. In the case ofthis judgment, when the accumulated PM quantity PMs becomes larger thanthe predetermined judgment value PMs0 for regeneration start, it isjudged that DPF regeneration is necessary.

When it is judged in this step S12 that sulfur purge is not necessary orthat DPF regeneration is not necessary, the DPF regeneration control orsulfur purge control is not performed and return is performed.

Moreover, when it is judged that sulfur purge is necessary and DPFregeneration is also necessary, step S13 is started.

In the case of the exhaust gas temperature raising control forregenerating the DPF in step S13, the exhaust gas temperature is raisedby performing post injection in accordance with the fuel injection of anengine or cutting EGR and control is performed so that the exhaust gastemperature flowing into the DPF 41 b enters a PM self-ignition regionand temperature region free from abnormal combustion (approx. 500° C.).In this temperature control, feedback control is performed so that thetemperature of exhaust gas flowing into the NOx occlusion reduction typecatalyst converter 42 becomes a temperature capable of performing sulfurpurge (approx. 600° C. to 650° C.) or higher while monitoring thetemperature detected by the temperature sensor 52 and adjusting the fuelquantity of post injection.

The PM accumulated in the DPF 41 b is forcibly burned and removed inaccordance with the exhaust gas temperature rise. Moreover, temperaturesof the DPF 41 b, the exhaust gas, and NOx occlusion reduction typecatalyst converter 42 are raised by the combustion heat of the PM andthe oxygen concentration of the exhaust gas passing through the DPF 41 bis decreased.

Furthermore, after performing the DPF regeneration control in this stepS13 for a predetermined time tdpf (e.g. 10 min to 15 min), it is judgedin step S14 whether sulfur purge is completed. When the sulfur purge isnot completed in this judgment, step S15 is started to perform thesulfur purge control but when the sulfur purge is completed, step S16 isstarted.

Whether the sulfur purge is completed in step S14 is judged inaccordance with whether a sulfur purge quantity integrated value Spucalculated in the next step S15 exceeds an accumulated sulfur quantitySsp calculated in step S11 (or predetermined limit value Ssp0). Thesulfur purge quantity integrated value Spu is set to zero as an initialvalue in step S11 or calculated in the following step S15. Moreover,when the sulfur purge quantity integrated value Spu exceeds theaccumulated sulfur value Ssp (or predetermined limit value Ssp0), it isjudged that sulfur purge is completed and step S16 is started withoutperforming the sulfur purge control in the next step S15.

Then, the sulfur purge control in step S15 performs feedback control ofmultistage injection including pilot injection and post injection sothat a predetermined oxygen concentration (or excessive air rate λ) isobtained by monitoring an oxygen concentration (or excessive air rate λ)while controlling post injection, intake air throttle, and EGR. Theoxygen concentration (or excessive air rate λ) is an oxygenconcentration (or excessive air rate λ) of exhaust gas flowing into theNOx occlusion reduction type catalyst converter 42, which is a valuedetected by a second exhaust gas concentration sensor 54. The oxygenconcentration (or excessive air rate λ) of the control target is assumedas a rich air-fuel ratio, preferably as a stoichiometric air-fuel ratio(theoretical air-fuel ratio). The air-fuel ratio state of the exhaustgas flowing into the NOx occlusion reduction type catalyst converter 42is brought into a rich air-fuel ratio state, preferably to astoichiometric air-fuel ratio state by the air-fuel ratio control toefficiently perform sulfur purge.

In the case of the air-fuel ratio control, oxygen is consumed byoxidizing HC and CO by an upstream-side oxidation catalyst 41 a or PM bythe DPF 41 b. Therefore, it is not necessary to realize a complete richair-fuel ratio or complete stoichiometric air-fuel ratio immediatelyafter an exhaust manifold 21 of an engine E. That is, even if theexcessive air ratio λ is a shallow rich state of 1.01 to 1.02, it ispossible to bring the NOx occlusion reduction type catalyst converter 42into a rich atmosphere or stoichiometric atmosphere in which sulfur canbe purged. Therefore, it is only necessary to perform control so thatthe oxygen concentration (or excessive air rate λ) detected by a secondexhaust gas concentration sensor 54 closest to the upstream side of theNOx occlusion reduction type catalyst converter 42 becomes a richair-fuel ratio, preferably a stoichiometric air-fuel ratio. Thereby, itis possible to restrain deterioration of fuel efficiency, dischargequantity of HC or CO to the atmospheric air, drivability (ride quality),and the like.

After performing the sulfur purge control for a predetermined time tsp,the sulfur purge quantity integrated value Spu is calculated from apreviously input sulfur purge quantity map and the like in accordancewith a catalyst temperature Ts calculated from temperatures detected bythe first and second temperature sensors 51 and 52, and engine speed Neshowing an operation state and load Q of an engine. The catalysttemperature Ts is calculated from temperatures detected by the first andsecond temperature sensors 51 and 52. The sulfur purge quantity map isbrought into map data so that a sulfur purge quantity can be calculatedfrom the engine speed Ne and load Q by using the catalyst temperature Tsand the like as a parameter. The sulfur purge quantity map is determinedby experimentally obtaining the sulfur purge quantity for unit time byusing catalyst temperature, time, space speed (exhaust gas flow rate),and S/V ratio as parameters and putting the data in order.

The predetermined time tsp for performing the sulfur purge control isset to one several-th of the time required to complete sulfur purge(such as 180 s to 300 s) or 2 s to 60 s. Thereby, it is possible toprevent a lot of sulfur from being purged and discharged at thebeginning of the sulfur purge control. Moreover, in the case of a dieselengine, it is possible to decrease the time for the stoichiometricair-fuel ratio state which is difficult to keep for a long time. Thepredetermined time tsp is a value set earlier than the control, which isobtained from a condition in which sulfur purge can be efficientlyperformed previously obtained from experiments.

In the next step S16, the accumulated PM quantity PMs of the DPF 41 b iscalculated from the pressure difference ΔP detected by a pressuredifference sensor 55 and the like. Then, in the next step S17, it isjudged whether sulfur purge and DPF regeneration are completed. Thisjudgment is performed in accordance with whether the sulfur purgequantity integrated value Spu exceeds the accumulated sulfur quantitySsp (or predetermined limit value Ssp0) and whether the accumulated PMquantity PMs becomes equal to or less than a predetermined judgmentvalue PMs1 for completing regeneration.

When it is judged that neither sulfur purge nor DPF regeneration iscompleted, step S13 is restarted but when it is judged that sulfur purgeand DPF regeneration are both completed, return is performed.

Therefore, steps S13 to S17 are repeated until both sulfur purge and DPFregeneration are completed. When the both are completed, step S18 isstarted. In step S18, return control to a normal lean state is performedand return is performed after the return control.

According to the exhaust gas purifying method and exhaust gas purifyingsystem 1 having the above configuration, regeneration of the DPF 41 baccording to forcible combustion of PM and sulfur purge of the NOxocclusion reduction type catalyst 42 are simultaneously performed.Therefore, because the temperature of the NOx occlusion reduction typecatalyst can be raised by using the heat produced due to the forciblecombustion of the PM. It is possible to minimize deterioration of fuelefficiency.

Moreover, by intermittently repeating sulfur purge control, it ispossible to prevent a lot of sulfur from being purged and discharged atthe time of the sulfur purge control. Furthermore, it is not necessaryto keep the air-fuel ratio state of exhaust gas in a rich air-fuel ratiostate or stoichiometric air-fuel ratio state for a long time.

Furthermore, because the sulfur purge control for bringing the air-fuelratio state of exhaust gas into a rich air-fuel ratio state orstoichiometric air-fuel ratio state is intermittently performed, it ispossible to efficiently perform sulfur purge while restrainingdeterioration of slip of HC, CO, H₂S, and the like and drivability.

Furthermore, by bringing the air-fuel ratio state of exhaust gas into astoichiometric air-fuel ratio state, the above advantage can be furtherexhibited.

Embodiments

FIG. 7 shows the excessive air rate λ, pressure difference ΔP betweenfront and rear of a DPF, catalyst temperature (bed temperature of NOxocclusion reduction type catalyst converter) Tn, and concentrations ofNOx and SO₂ at the downstream side of an exhaust gas purifying systemwhen performing sulfur purge in accordance with the control flow shownin FIG. 6 by using the exhaust gas purifying system shown in FIG. 2.

According to FIG. 6, by intermittently repeating the sulfur purgecontrol for decreasing the excessive air rate λ immediately before a NOxocclusion reduction type catalyst converter to 1.01 to 1.02 whileperforming the regeneration control of a DPF, it is found that thecatalyst temperature Tn rises at the time of the sulfur purge controland SO₂ is discharged. Moreover, the quantity of the SO₂ to bedischarged at one time is decreased because the sulfur purge control iskept for a short time.

Furthermore, because the pressure difference ΔP between front and rearof the DPF is lowered, it is found that combustion of PM is progressed.In FIG. 6, the sulfur purge control is performed for 1 min after a 3 minstop period and this cycle is repeated. Furthermore, in the range ofFIG. 6, neither sulfur purge nor DPF regeneration is completed.

1. An exhaust gas purifying method using an exhaust gas purifying systemhaving a DPF and an NOx occlusion reduction type catalyst in order fromthe upstream side in an exhaust passage of an engine and a controlmeans; said control means comprising; an exhaust gas component detectionmeans to detect the oxygen concentration and NOx concentration of theexhaust gas passing through the NOx occlusion reduction type catalyst; aDPF regeneration control means for controlling regeneration of the DPF;a NOx catalyst regeneration control means for recovering the NOxocclusion ability of the NOx occlusion reduction type catalyst; and asulfur purge control means for controlling the sulfur purge of the NOxocclusion reduction type catalyst; wherein, when it is judged that bothregeneration of the DPF and sulfur purge of the NOx occlusion reductiontype catalyst are necessary, the DPF regeneration control for raisingthe temperature of the DPF is performed and the sulfur purge control todecrease the oxygen concentration in the exhaust gas flowing into theNOx occlusion reduction type catalyst is intermittently repeated.
 2. Theexhaust gas purifying method according to claim 1, wherein the air-fuelratio state of the exhaust gas flowing into the NOx occlusion reductiontype catalyst is brought into a stoichiometric air-fuel ratio state inthe sulfur purge control.
 3. An exhaust gas purifying system having aDPF and a NOx occlusion reduction arranged in order from the upstreamside in an exhaust passage of an engine, and a control means; saidcontrol means comprising; an exhaust gas component detection means todetect the oxygen concentration and NOx concentration of the exhaust gaspassing through the NOx occlusion reduction type catalyst: a DPFregeneration control means for controlling regeneration of the DPF; aNOx catalyst regeneration control means for recovering the NOx occlusionability of the NOx occlusion reduction type catalyst; and a sulfur purgecontrol means for controlling the sulfur purge of the NOx occlusionreduction type catalyst; wherein when it is judged that bothregeneration of the DPF and sulfur purge of the NOx occlusion reductiontype catalyst are necessary, said control means performs the DPFregeneration control for raising the temperature of the DPF and executesintermittently the sulfur purge control to decrease the oxygenconcentration in the exhaust gas flowing into the NOx occlusionreduction type catalyst,.
 4. The exhaust gas purifying system accordingto claim 3, wherein the air-fuel ratio state of the exhaust gas flowinginto the NOx occlusion reduction type catalyst is brought into astoichiometric air-fuel ratio state in the sulfur purge control.